This peach trouble is known only in the eastern and central United States; so far it has not been reported in any other region of the globe. Not only is its range limited but its history is rather brief, and its importance has prompted scientific writings only during the past decade. But in the short history of Black Spot, or shot-hole as it is frequently called, it has assumed a very prominent role in the culture of stone-fruits in the humid regions of the South. It is less important in the North and East. In Missouri it is rated as one of the worst 'of fruit diseases. The losses induced by the Black Spot patho-gene are not easily reckoned. But in Missouri it has been found that from 1 to 10 per cent of the Elberta peach fruits is injured in well cared for orchards, and from 25 to 75 per cent in poorly managed orchards. All affected fruits are graded second class. Furthermore, such fruits do not ship well, often being completely rotted in transit as a result of rot-fungi which gain entrance through Black Spot lesions. The injury is not confined to the fruit alone, but foliage is attacked. Diseased foliage is less efficient than healthy leaves, and so the vitality of the tree suffers. This also results, in the event of early spring infection, in the reduction in size and quality of the fruit. Severe leaf injury also results in a reduction in the vitality of the fruit - buds and even prevents the formation of the normal number, so that the next year's crop of peaches will be materially lightened. Young trees suffer severely; when badly diseased, they become stunted and in some cases are permanently injured.
It should be stated that Black Spot affects not only the fruit, foliage and twigs of the peach, but also these same organs of the apricot, nectarine and plum, making four prominent and important stone-fruit trees in the category of hosts for the Black Spot pathogene. This fact adds to the economic aspect of this disease. Of the peaches the Elberta is particularly liable to extensive damage. Likewise the Champion, Carman, Alton and others are very susceptible.
The disease is more likely to be found on young Elberta trees than on peaches of other ages and varieties, although practically all kinds are affected to a varying degree. The leaves (Fig. 81), fruits and twigs show symptoms of the disease.
On the leaves a shot-hole effect similar to the Leaf Blight of plums and cherries is produced (Fig. 81). The first evidence of the trouble is a Leaf Spot; hence the disease is at times called bacterial Leaf Spot. The spots at the beginning are mere specks, grayish in color, angular in form, and take on a water-soaked aspect. Later, they become brown or purple-brown, or even scarlet, although they are finally dark-brown. Mature spots ordinarily do not measure more than one-fifth of an inch in diameter; many are smaller, but two or more may coalesce so that large areas are involved (Fig. 81). In later stages the affected tissue contracts, dries and falls away, leaving a more or less circular hole (Fig. 81). Several such holes in a leaf give it the appearance of having been riddled with gun-shot, whence the name shot - hole (Fig. 81). Badly diseased leaves fall prematurely; in this way an infected tree may lose from 15 to 75 per cent of its foliage by August. Not all leaves fall at once, but one after another, until finally only a few young leaves remain at the tip of each twig. In this condition the affected tree is characteristic in its appearance.
Fig. 81. - Black Spot lesions on peach-leaves. Note the shot - hole effect.
The lesions on fruits in their early stages are similar to those described for the leaves. Soon, however, the skin is ruptured at affected places, resulting in the production of numerous angular cracks. While the diseased areas are very small - never more than one-tenth of an inch in diameter - they are often so numerous that the crevices run together, thus forming a network of fissures some of which may be an inch in extent. Some growers speak of this condition as bacterial-crack. Black Spot does not usually become evident on the fruit until about the middle of May.
The disease on the twigs shows itself in the form of black spots or cankers; this phase has received the names Black Spot, black-tip and bacterial cankers. These lesions are found abundantly in May and June, or even earlier. A single spot develops as follows: surrounding a lenticel there first appears a water-soaked area which bulges out somewhat. As the spot enlarges it elongates, and at maturity extends from one-half to two inches up and down the shoot and from one-half to twothirds of the way around. Older lesions are brownish or purplish brown, then dark - brown, and ultimately are purplish black or jet black and sunken to some extent. Occasionally open cankers form on peach twigs, but, like those on the apricot and nectarine, are much less common than on the plum.
This is one of the many bacterial diseases which affect plants; the pathogene is known as Bacterium Pruni. The causal relationship between Bad. Pruni and Black Spot or shot - hole has been known nearly as long as has the disease itself.
The black spots or cankers on the twigs are the chief sources of the inoculum in the spring. In other words, the bacteria pass the winter largely on the twigs in the lesions of the previous year's formation. They may also hibernate in the buds in some fashion, and in fallen leaves, if the same are well protected from the sun and air, but these are not important sources of the inoculum in the spring. Rain, dew and insects carry the bacteria from their hibernating quarters to the lower surface of young leaves, in the month of May or later, where they gain entrance through the stomata. The twigs are penetrated by way of the young lenticels. Moisture is essential for this process. Within a week, more or less depending upon the temperature and moisture conditions, and upon the organ attacked, evidences of the disease are visible to the naked eye. In this interim, that is, from the time the peach is entered until the disease is evident, the bacteria multiply rapidly and secrete a solvent which soon breaks down the cell walls, giving the pathogene a good food-supply. In a short time a small cavity is formed beneath the surface of the spot in which many bacteria obtain. These finally escape to the surface, through stomata or crevices, where they lie in sticky masses. They are disseminated to other points, and new infections arise on the current season's twig - growth. With the advent of autumn the bacteria become dormant and remain so until spring, when the cycle begins anew.
The weather has a rather marked effect upon the severity of the Black Spot disease. Ordinarily an outbreak may not be expected until May; however, if the spring weather is warm and damp, infections appear as early as April 1. A temperature between 68° and 82° Fahrenheit is most favorable to the bacteria. Cold weather not only checks the activities of the bacteria, but also of the insects which disseminate the bacteria. Heavy, driving rains of short duration, even if accompanied by a favorable temperature, are not conducive to the spread of the pathogene; for such rains carry the bacteria to the ground. And should the showers be followed by sunshine and breezes, it is unlikely that a serious outbreak will ensue on account of the fact that the susceptible parts would thus be quickly dried off and the bacteria on the surface would die. Heavy dews, if accompanied by proper temperatures and shaded locations, are favorable to the growth of the bacteria. Slight injury from Bact. Pruni results in dry seasons, especially if the spring has been cold.
Reasoning from the facts and phenomena recorded above, treatment may be followed along several lines as tabulated below.
(1) The disease affects the peach, plum, nectarine and apricot. Growers of all four fruits may find the problem of control more complicated than where, for example, only peaches are grown. A few old and comparatively worthless trees of plums, apricots or nectarines may be a constant source of trouble. These should be removed.
(2) The growing of resistant varieties offers little relief at present.
(3) The 'twigs, fruits and leaves are affected. The injury may be serious on any or all of these organs. The twigs furnish hibernating quarters for the bacteria. Careful pruning should include the removal of diseased twigs, in order to destroy the source of the inoculum.
(4) The disease is bacterial, and the general rule that bacterial diseases are not directly controlled by spraying seems to apply to Black Spot. It has been found, however, that spraying has an indirect effect in controlling disseminating insects. A special schedule of applications is not warranted.
(5) Orchards which are properly managed, i.e. properly pruned, cultivated, fertilized and sprayed, invariably suffer less than those otherwise handled.
(6) The pathogene is spread in nursery stock, consequently in planting new orchards use stock bought only from reliable nurserymen. Younger trees suffer more within five years after setting than do older trees that are in good condition.
Rolfs, F. M. A bacterial disease of stone fruits. Cornell Univ. Agr.
Exp. Sta. Memoir, 8: 381-436. 1915. Rorer, J. B. A bacterial disease of the peach. Mycologia, 1: 23 - 27.1909.